 on the Tanks Focus Area About the focus areas The U.S. Department of Energy’s Office of Environmental Management has established an integrated approach for addressing waste issues based on problem, or focus, areas. The focus areas are subsurface contaminants; mixed waste characterization, treatment, and disposal; radioactive tank waste remediation; deactivation and decommissioning; and plutonium stabilization. Three crosscutting technology areas support the focus areas: characterization, monitoring, and sensor technology; efficient separations and processing; and robotics. This issue of Initiatives continues its spotlight on the Tanks Focus Area that was begun in the October/December issue when Initiatives featured TFA’s characterization and retrieval activities. Following are some of the significant accomplishments and directions taken by TFA to meet the needs of sites to treat and immobilize tank waste before tank closure.
Tanks 20 and 17 at the Savannah River Site are closed. In late July 1997, staff at the Savannah River Site in South Carolina achieved, with TFA’s help, the first closure of a radioactive waste storage tank in the U.S. Department of Energy’s nuclear complex. The objective of the closure was to trap the residual radioactivity inside the tank in a rock-hard monolith that would be highly resistant to leaching and earthquakes. The staff accomplished the closure by pouring grout into the tank through seven risers to immobilize residual sludge. Dry cement powder was added to absorb moisture and help bind the sludge. The tank was next filled with a low-strength material. When the tops of the sidewalls were reached, the staff poured a high-strength material to prevent intrusions into the dome space and risers. The last step accomplished in August was capping the distribution pipes into the tank. Waste removal activities were then begun in Tank 17. On December 15, Tank 17 was closed, making it the second closure of a high-level radioactive waste tank at the site and the second ever in the nation. Treating tank waste TFA develops and tests separation technologies for removing high-level radionuclides from low-level and hazardous waste streams. If the relatively small volume of highly radioactive waste can be separated from the larger volume of nonradioactive chemicals (e.g., sodium hydroxide, aluminum, and water), only the highly radioactive part would require vitrification. The low-activity fraction could be disposed of with less risk to workers and the public and less expense to taxpayers. Transuranic extraction is demonstrated at Idaho National Engineering and Environmental Laboratory. In FY96, in a joint effort with INEEL’s High-Level Waste Program, which is funded by DOE’s Office of Waste Management (EM-30), TFA demonstrated the TRUEX process for removing transuranic elements from INEEL’s acidic tank waste. By successfully separating transuranics from the waste, the TRUEX process resulted in waste with a radioactivity of 0.12 nanocuries per gram (nCi/g), nearly 100 times less than the U.S. Nuclear Regulatory Commission’s criterion for Class A low-level waste of 10 nCi/g. In August 1997, this transuranic solvent extraction technology was demonstrated on waste from Tank WM-183 at INEEL. The process reduced the actinide level from 540 nCi/g to 0.9 nCi/g, again well below the 10 nCi/g limit set by NRC for waste to be classified as low level. The TRUEX process dissolves CMPO (a complexing agent) in an inorganic solvent, resulting in a carbon-based solvent containing tributyl phosphate and a paraffinic hydrocarbon. This compound contacts radioactive tank waste in highly efficient countercurrent extraction equipment called centrifugal contactors. The CMPO binds with the 3+, 4+, and 6+ valence transuranic elements and holds them in the organic phase (solvent). The inert materials, which now contain much less than 10 nCi/g of activity from the transuranics, are trapped in the nonsolvent phase. These two phases separate from each other, similar to oil and water. The transuranics are stripped from the solvent, which can be recycled and reused in the process. The inert materials can then be disposed of as Class A low-level waste.
During the fourth test of Oak Ridge’s cesium removal system in May 1997, approximately 15,000 gallons of Melton Valley Storage Tank supernate was processed during a three-column run, during which approximately 631 curies of cesium-137 was removed. The sorbent from all three columns was sluiced, dried, and moved to on-site temporary storage at Oak Ridge. Permanent disposal is planned at the Nevada Test Site. TFA has also demonstrated how the cesium removal system equipment can be decontaminated and transported to another location, important steps in making the system useful to DOE tank sites. CST vitrification testing begins. TFA is also demonstrating how vitrification can be used to dispose of the cesium-loaded CST from the cesium removal system that was demonstrated at Oak Ridge Reservation. In August, part of the spent CST was shipped to the Savannah River Technology Center for vitrification testing. During crucible tests, glass formers were added to the CST resin to make glass with CST loading of 52 wt %. The CST/glass former mixture was heated to 1,150ºC and held at this temperature for four hours in a platinum crucible. The resulting glass had no visual signs of crystallization and readily poured from the crucible. Following this successful crucible test that produced a quality glass waste form, radioactive CST and glass formers were added to a pilot-scale, joule-heated melter. After approximately 80 hours of operation, 20 kilograms of glass was produced. This effort demonstrated that CST can be effectively vitrified with available processing systems to produce a quality waste form. By proving the viability of caustic recovery, TFA paves the way for industry participation. TFA successfully completed its caustic recycle work for FY97. TFA is now working with the Federal Energy Technology Center so the caustic recovery work can proceed through industry participation. In May 1997, TFA started conducting tests of a pilot-scale caustic recovery and recycle process to remove sodium hydroxide from tank waste. Years ago, DOE added sodium hydroxide to tank waste at a number of sites to change the pH of waste from acidic (which corrodes carbon steel tanks) to basic. By removing sodium hydroxide, the volume of high-level tank waste will be greatly reduced, avoiding the treatment of all tank waste as high level. An electrochemical-based process was used on waste simulant that matched solutions stored in Savannah River Site tanks. For two tests, an organic membrane, Nafion® Type 350, was placed in an electrochemcial cell to form cathode and anode compartments. Waste solution was fed into the anode, and a dilute sodium hydroxide solution was fed into the cathode. Sodium ions moved across the membrane into the cathode, reducing the concentration of water at the cathode and producing hydroxide ions and a stronger caustic solution. Liquid samples taken from the cathode and anode were analyzed to calculate sodium and hydroxide concentration to determine electrical efficiency and transport rates. In addition to the tests using SRS simulant, five bench-scale tests were conducted using actual SRS radioactive waste and Nafion® and Ceramatec® membranes. The tests showed
For more information on TFA’s pretreatment activities, contact Phil McGinnis,
ORNL For more information on TFA’s immobilization activities, contact Bill
Holtzcheiter, WSRC For more information on TFA’s closure activities, contact Thomas Brouns,
Technical Technical highlights from the Tanks Focus Area are updated twice a month and published on the Internet at http://www.pnl.gov/tfa/hilight. To receive an e-mail version of the highlights, follow the instructions given at the bottom of the Web page. |
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Closing tanks